The reality of shark (elasmobranch) attacks and a pervasive fear of shark attacks in the modern world combine to create a great need for effective shark repellents. Effective shark repellents are also needed in the commercial fishing industry.
Elasmobranchs represent a significant problem in the commercial fishing industry. Elasmobranchs are often inadvertently caught on fishing hooks and tackle directed at other more commercially valuable kinds of fish. This inadvertent catching of elasmobranchs (or other non-valued fish) is called “by-catch.” As many as 100 million elasmobranchs are killed each year as by-catch. This loss of life has resulted in a real threat to several shark species. Currently, as many as 80 species of shark are considered threatened with extinction.
Further, when elasmobranchs are caught as by-catch, fishing operations receive no return on their investment since the shark is caught on a hook that might have otherwise brought in a marketable fish. Additionally, the fishing tackle on which a shark is caught often must be cut loose for the safety of those working on the fishing vessel causing a loss of both equipment and time.
Longlining is a commercial fishing method that suffers significant losses from shark by-catch. Longlining uses multiple baited individual fish hooks with leaders strung at intervals along an often very long (2-3 mile) main fishing line. Longline fishing operations routinely target swordfish and tuna. The longline hooks and bait, however, are not selective and elasmobranchs are sometimes caught in greater numbers than the intended target catch. The result is great loss of life in elasmobranchs and significant financial losses in the longline industry. Elasmobranchs cause additional losses in the longline fishing industry by scavenging marketable fish caught on longlines before the fish may be retrieved for processing. This problem also applies to the commercial trawling industry.
There has been a long-felt need for methods and devices to deter elasmobranchs from commercial fishing lines and nets. Attempts in the middle of the twentieth century were made to protect trawl nets with electric discharge devices. Nelson, “Shark Attack and Repellency Research: An Overview,” Shark Repellents from the Sea ed. Bernhard Zahuranec (1983) at p. 20). Nevertheless, no commercially effective repellent has been made available for reducing shark by-catch in the commercial fishing industry or for reducing loss of valuable fish or fishing tackle to shark predation.
An effective shark repellent would not only be valuable to the fishing industry but also would be valuable for protecting humans from shark attacks. An effective repellent has yet to be marketed for limiting the risk of shark attacks faced by humans exposed to elasmobranchs. Over the last 50 years antishark measures employed to protect humans from sharks have included electrical repellent devices (Gilbert & Springer 1963, Gilbert & Gilbert 1973), acoustical playbacks (Myrberg et al. 1978, Klimley & Myrberg 1979), visual devices (Doak 1974) and chemical repellents (Tuve 1963, Clark 1974, Gruber & Zlotkin 1982). None of these procedures proved satisfactory in preventing shark attacks. (Sisneros (2001)). As such, the long felt need for an effective repellent has not been satisfied.
Researchers have historically used several bio-assays to determine if a repellent evokes a flight response in shark. One such bio-assay measures the effect of a repellent on a shark that is immobilized in “tonic immobility.” Tonic immobility is a state of paralysis that typically occurs when a shark is subject to inversion of its body along the longitudinal axis. This state is called “tonic,” and the shark can remain in this state for up to 15 minutes thereby allowing researchers to observe effects of repellents. After behavioral controls are established, an object or substance that has a repelling effect will awaken a shark from a tonic state. Researchers can quantify the strength of a repellent effect from these studies.